Optically clear commercial poly(alkylene carbonate) binder materials, such as poly(ethylene carbonate) (PEC) and poly(propylene carbonate) (PPC), have demonstrated utility in air-gap formation for electrical/optical interconnects, microelectromechanical systems (MEMS) device fabrication, microfluidics, and micro-reactor applications because of their photoimageability in the presence of a photoactive additive. For example, Unity® 2203P, which contains a poly(alkylene carbonate), can be patterned by UV exposure (typically at 365 nm) in the presence of a suitable additive. However, retention of pattern fidelity and feature resolution is limited due to polymer flow issues (e.g., rounded features, slanting side walls) in post-exposure development. These thermal flow properties are attributed to the low glass transition temperature (Tg, 40° C.) of the base polymer (PPC) which is significantly lower than the development temperature. Thus, there is a need to develop new photoimageable polymers which are formed from readily available monomers and exhibit the following properties: (i) high Tg (Tg≧80° C.), (ii) soluble in common process solvents, (iii) have sufficient Mw (at least 15,000) to attain formulation viscosity required for thin film generation by spin or spray coating and have mechanical strength, (iv) decompose cleanly at low temperatures (<200° C.) in the presence of a photoactive additive, and (v) leave essentially no residue after post-exposure thermal development. The low residue requirement is important in cases where cleaning is not possible, e.g., encapsulated microchannels generated from decomposition of sacrificial materials covered by an overcoat.
U.S. Pat. No. 6,743,570 B2 ('570 patent) discloses a method of using heat-depolymerizable polycarbonate, such as poly(cyclohexene carbonate) to create a nano-fluidic device. In this method, areas of polycarbonate exposed to e-beam were removed by immersion in isopropanol, with optional plasma cleaning in a UV-ozone cleaner. In a later part of the device fabrication, after depositing a capping layer, the underlying heat-depolymerizable polycarbonate was removed by baking at temperatures above 300° C. for 30 min or longer. No photoactive component was mentioned in the disclosure. The poly(cyclohexene carbonate) was prepared from alternating copolymerization of cyclohexene oxide with carbon dioxide using (BDI)ZnOR (R=Ac, Me) catalysts in accordance with the procedures described by Cheng et al., J. Am. Chem. Soc. 1998, 120, 11018-11019; also see J. Am. Chem. Soc. 2003, 125, 11911-11924. Various other catalyst systems have also been used for the preparation of polycarbonates by alternating copolymerization of epoxide with carbon dioxide, see Catal. Sci. Technol. 2012, 2, 2169-2187 and references cited therein.
It should be noted that the post e-beam immersion in isopropanol processing as disclosed in '570 patent may not be suitable in many applications where the polycarbonate is intended for removal after an overmolding step, such as in a typical semiconductor fabrication. Furthermore, the alternative harsh baking conditions (higher than 300° C. for a period in excess of 30 minutes) may not be compatible with expected throughput and thermally sensitive components in a semiconductor device.
U.S. Pat. No. 6,586,154 B1 ('154 patent) discloses a series of photoresist polymer compositions which are polycarbonates derived from various polycyclic diols and carbon dioxide. It should be noted that the disclosures of '154 patent appear to require a wet development step after a post-exposure bake, as disclosed therein several of the examples included a 40-second tetramethylammonium hydroxide (TMAH) development step.
U.S. Patent Application Publication No. 2004/0132855 discloses a series of photodefinable polymers, which include polynorbornenes, polycarbonates, polyethers and polyesters. As disclosed therein, residues formed from certain decomposable materials, such as polyimide, polynorbornene, and polycarbonate are removed by plasma reactive ion etch (RIE). This aspect of the process may not be suitable in cases where the microchannel is encapsulated by an overcoat.
Accordingly, there is still a need for developing polymer compositions that can be dry developed with essentially no residue left behind.
Other objects and further scope of the applicability of the present invention will become apparent from the detailed description that follows.